- Ondansetron is well absorbed from the gastrointestinal tract and undergoes some first-pass metabolism. Mean bioavailability in healthy subjects is approximately 56% and bioavailability is also slightly enhanced by the presence of food and at higher doses but unaffected by antacids.
- Ondansetron is distributed with volume of distribution of 1.9-2.6 L/Kg, indicating that much of the drug is taken up by body tissues. Circulating drug also distributes into erythrocytes. Plasma protein binding is 70% and crosses membranes readily.
- Biotransformation pathways involve the primary metabolic pathway is hydroxylation on the indole ring followed by subsequent glucuronide or sulfate conjugation.
- Ondansetron is excreted in the urine (approximately 65%) and feces (35%) after extensive hepatic metabolism. The plasma half-life is 3.5 hours.
Pharmacodinetics: Ondansetron is a potent, highly selective 5HT3 receptor-antagonist. Its precise mode of action in the control of nausea and vomiting is not known. Chemotherapeutic agents and radiotherapy may cause release of 5HT in the small intestine initiating a vomiting reflex by activating vagal afferents via 5HT3 receptors. Ondansetron blocks the initiation of this reflex. Activation of vagal afferents may also cause a release of 5HT in the area postrema, located on the floor of the fourth ventricle, and this may also promote emesis through a central mechanism. Thus, the effect of ondansetron in the management of the nausea and vomiting induced by cytotoxic chemotherapy and radiotherapy is probably due to antagonism of 5HT3 receptors on neurons located both in the peripheral and central nervous system. The mechanisms of action in post-operative nausea and vomiting are not known but there may be common pathways with cytotoxic induced nausea and vomiting.
Dosage & Administration
Chemotherapy-induced Nausea and Vomiting-
Adults/Geriatric/Child of 12 years or over:
- Highly emetogenic cancer chemotherapy: 30 ml (24 mg) Ondansetron Oral Solution administered 30 minutes before start of emetogenic chemotherapy.
- Moderate emetogenic cancer chemotherapy: 10 ml (8 mg) Ondansetron Oral Solution administered 30 minutes before start of emetogenic chemotherapy. A further 10 ml dose should be administered after 8 hours of the first dose. One 10 ml dose should be administered twice a day (every 12 hours) for 1-2 days after completion of chemotherapy.
Pediatric (4-11 years): 5 ml (4 mg) Ondansetron Oral Solution should be taken 30 minutes before the start of chemotherapy. The other 2 doses should be taken 4 and 8 hours after the first dose. Then 5 ml oral solution should be administered 3 times a day (every 8 hours) for 1-2 days after completion of chemotherapy.
Radiotherapy induced Nausea and Vomiting-
Adults/Geriatric/Child of 12 years or over:
- The recommended oral dosage: 10 ml (8 mg) Ondansetron Oral Solution 3 times daily.
- For total body irradiation: 10 ml (8-mg) Ondansetron Oral Solution should be administered 1 to 2 hours before each fraction of radiotherapy administered each day.
- For single high-dose fraction radiotherapy to the abdomen: one 10 ml Ondansetron Oral Solution should be administered 1 to 2 hours before radiotherapy, with subsequent doses every 8 hours after the first dose for 1 to 2 days after completion of radiotherapy.
- For daily fractionated radiotherapy to the abdomen: 10 ml (8-mg) Ondansetron Oral Solution should be administered 1 to 2 hours before radiotherapy, with subsequent doses every 8 hours after the first dose for each day radiotherapy is given.
Postoperative Nausea and Vomiting-
- Adults/Geriatric/Child of 12 years or over: 20 ml (16 mg) Ondansetron Oral Solution 1 hour before induction of anesthesia
Pregnancy & Lactation
Reproduction studies have been performed in pregnant rats and rabbits at daily oral doses up to 15 and 30 mg/kg per day, respectively, and have revealed no evidence of impaired fertility or harm to the fetus due to Ondansetron. There are, however, no adequate and well-controlled studies in pregnant women. Ondansetron is excreted in the breast milk of rats. So caution should be exercised when Ondansetron is administered to a nursing women.
Precautions & Warnings
Use in Special Populations
Dosage Adjustment for Patients With Impaired Hepatic Function: In patients with severe hepatic impairment, a single maximal daily dose of 8 mg to be infused over 15 minutes beginning 30 minutes before the start of the emetogenic chemotherapy is recommended.
Little information is available about dosage in pediatric patients 4 years of age or younger.
Dosage adjustment is not needed in patients over the age of 65.